Lean burning internal combustion engines, including modern diesel engines, produce significant nitrogen oxides (NOx). Because of health risks associated with NOx, the U.S. Environmental Protection Agency places regulatory limits on acceptable levels of NOx emissions.
To reduce environmental impacts and to meet regulatory air pollution limits for diesel engines, manufacturers of diesel engines are adopting exhaust after-treatment systems that significantly reduce or eliminate undesirable emissions. After-treatment systems, for example, diesel particulate filters, such as catalyzed soot filters, trap diesel particulate matter and reduce emissions. Selective catalytic reduction (SCR) is a technology for reducing NOx in engine exhaust. SCR is the leading technology being used to meet emission regulations for 2010 and beyond. While NOx encompasses a number of different compounds, for practical applications it is frequently enough to consider only NO and NO2, and NOx is sometimes used to refer specifically to these compounds.
SCR systems catalytically react exhaust stream NOx with other compounds to produce diatomic nitrogen and water. A typical SCR system receives engine exhaust into a chamber containing a suitable catalyst. Small quantities of a reductant are injected into the exhaust upstream of a catalyst. NOx reductants include, for example, anhydrous ammonia, aqueous ammonia, or urea. The reductant, cooperatively with the catalyst, reduces NOx into more benign compounds. SCR systems used in modern diesel trucks use a reductant referred to as diesel exhaust fluid (DEF), and standardized as ISO 22241. DEF is an aqueous urea solution of 32.5% high purity urea and 67.5% deionized water. DEF is metered or injected into the exhaust stream and thermally decomposes to produce ammonia, which, in the presence of the catalyst reacts with NOx in the exhaust, producing nitrogen and water.
Optimally, the quantity of reductant injected is sufficient to react substantially all of the NOx in the exhaust. However, if too much reductant is injected into the exhaust, excess reductant, e.g., ammonia, will exit the SCR without reacting. Reductant that exits the SCR without reacting with NOx is referred to generally as ammonia slip. Typically, an ammonia slip catalyst is provided downstream, to prevent ammonia from exiting the tail pipe. The ammonia slip is an undesirable emission and is wasteful of the consumable reductant. On the other hand, if too little reductant is injected into the SCR then undesirable levels of unreacted NOx will exit the SCR. NOx that exits the SCR is generally referred to as NOx slip. In an optimal SCR system ammonia slip and NOx slip are minimized.
The composition and temperature of the incoming exhaust stream varies, and optimal reductant dosing is therefore a transient target. For example, in diesel engines used for Class 8 vehicles the engine operating parameters change due to changing engine load, changes in environmental conditions, and the like. The optimal rate of reductant dosing varies significantly during operation of the engine. Therefore reductant dosing is typically actively controlled, based on measured conditions, for example, by reducing the dosing if ammonia slip is detected or increasing the dosing if NOx slip is detected. It would be beneficial to predict the onset of ammonia slip and NOx slip, and to adjust reductant dosing to prevent (or mitigate) slip.